phew

A hex-encoded, gzipped encoder/decoder for spew.

When debugging go programs spew offers a wealth of information, but it can also be useful in production for capturing critical information about the execution of programs under real-world conditions.

The latter becomes somewhat challenging, however, since the spew pretty-printer deviates from the standard log format where everything fits on a single line. When printing large structs, the output can span several hundred or thousands of lines, obliterating the readability of the production logs.

Consider the following struct:

type VeryLargeThing struct {
    Number      uint32
    LotsOfBytes []byte
}

Calling spew.Sdump on an instance of VeryLargeThing might resemble:

(main.VeryLargeThing) {
 Number: (uint32) 42,
 LotsOfBytes: ([]uint8) (len=8 cap=8) {
  00000000  62 69 67 20 64 61 74 61                           |big data|
 }
}

Often times we don't necessarily need to observe the spewed output, but rather we would like the information to be availabe if need to investigate an issue. However, embedding the above Sdump can cause production logs to become bloated, even if there is no active investigation.

Enter phew: instead of logging the pretty-printed struct directly, we log hex-encoded, gzipped output of spew so that the output can be unpacked later for evaluation.

As a drop in replacement, encoding the same struct with phew.Sdump produces the following output, which can be easily embedded in a single log line:

1f8b08000000000000ffd2c84dccccd30b4b2daaf4492c4a4f0dc9c8cc4bd754a8e652f02bcd4d4a2db252d028cdcc2b3136d2543031d2e152f0c92f29f64f73aa2c492db652d0888e05495a682a68e4a4e6d95a28242716d85a80752b184081828299918299a58299b98291818299898299a1823998c40d6a9232d31552124b126bb8146ab96a01010000ffff2ee62bb7a3000000

To inspect a phewed object at a later point, one can use the phew decode CLI command, which yields the original, spewed output:

$ phew decode 1f8b08000000000000ffd2c84dccccd30b4b2daaf4492c4a4f0dc9c8cc4bd754a8e652f02bcd4d4a2db252d028cdcc2b3136d2543031d2e152f0c92f29f64f73aa2c492db652d0888e05495a682a68e4a4e6d95a28242716d85a80752b184081828299918299a58299b98291818299898299a1823998c40d6a9232d31552124b126bb8146ab96a01010000ffff2ee62bb7a3000000
(main.VeryLargeThing) {
 Number: (uint32) 42,
 LotsOfBytes: ([]uint8) (len=8 cap=8) {
  00000000  62 69 67 20 64 61 74 61                           |big data|
 }
}

CLI Installation

Using go get:

go get github.com/cfromknecht/phew

From source:

git clone git@github.com:cfromknecht/phew.git
cd phew
go install ./cmd/phew